SUMMARY Our long-term goal is to investigate the therapeutic potential and the underlying molecular mechanisms of maspin in prostate cancer (PC) prognosis and treatments. Maspin, a highly conserved protein throughout evolution, belongs to the serine protease inhibitor (serpin) superfamily. Human maspin is expressed specifically in epithelial cells of several organs. In cancer, maspin correlates with better differentiated phenotypes and better prognoses. Accumulated experimental evidence demonostrates a tumor suppressive role of maspin at the steps of invasion and metastasis. Maspin reexpression in prostate tumor cells induces redifferention. Furthermore, maspin specifically sensitizes tumor cells, but not normal cells, to induced apoptosis. Interestingly, however, in our preliminary studies, a paradoxical role of maspin was observed: maspin enhanced tumor cell resistance to stress stimuli. A better understanding of the underlying moleuclar mechanisms of maspin is clearly called for in order to explore its therapeutic potential. It was realized only recently that maspin is the most deviant member in the serpin superfamily and has a unique conformational plasticity. This new revelation helps explain several lines of evidence generated from our lab including that maspin acts as a serine protease inhibitor-like molecule to inhibit serine protease-like targets such as pro-urokinase type plasminogen activator (uPA) and histone deacetylase 1 (HDAC1). The effect of maspin on HDAC1 is tied to its apoptosis-sensitizing effect. It is worth noting that despite the significant success in recent years in the synthesis of small molecular weight HDAC inhibitors, maspin is the only endogenous polypeptide HDAC inhibitor identified thus far. In the meantime, intracellular maspin has been shown also to interast with heatshock protein 90 (Hsp90) and glutathione S-WUDQVIHUDVH[unreadable][unreadable]LVRIRUP[unreadable][unreadable]*67 [unreadable][unreadable][unreadable]While maspin increases cellular GST activity, *67 [unreadable]seems to be required in maspin-mediated HDAC1 inhibition. Interestingly, maspin inhibited HDAC1- mediated Hsp90 deacetylation, leading to cell resistance to stress imposed by Hsp90 inhibitors. Taken together, our data led to a Novel Hypothesis that maspin may regulate the balance of survival and death of prostate epithelial cells by inhibiting HDAC1-mediated deacetylation of both histones and Hsp90, in a *67 -dependent manner. Our hypothesis suggests, for the first time, a biochemical model that links HDAC1, Hsp90 and *67 to maspin. It further suggests that by inhibiting HDAC1 in a GSTS-dependent manner, maspin may significantly incapacitate Hsp90-dependent oncogenic/survival signaling, strengthen the redox circuitry to neutralize reactive oxygen species generated through hypoxia or oxidative stress, and potentiate the cells to readily commit self-elimination when apoptosis executing signals are emitted in the tissue microenvironment. To test this hypothesis, we will address four Specific Aims. Aim 1: To Investigate the Maspin Effects on HDAC1 in Transcription Regulation and Hsp90 Activity. Aim 2: To Investigate the Effect of *67 on Maspin Activity. Aim 3: To Investigate the Maspin Effects on Prostate Epithelial Drug Sensitivity and Resistance. Aim 4: To Validate the Maspin Molecular Partnerships as Markers for PC Diagnosis and Prognosis. The loss of maspin and *67 marks poor differentiation and poor prognoses of PC. On the other hand, both HDAC1 and Hsp90 are targeted by synthetic inhibitors in experimental cancer therapies. Our hypothesis suggests, for the first time, a biochemical model that links HDAC1, Hsp90 and *67 to maspin. The dynamics of these complex partnerships is the focus of the current application. The expected resutls are likely to establish the clinical relevance and functional significance of our novel hypothetical model, which is likely to have a paradigm-shifting impact regarding the therapeutic potential of maspin in PC prognosis and treatment.